Method and apparatus for transmitting reference signals in uplink multiple input multiple output (MIMO) transmission

ABSTRACT

The present invention relates to a wireless communication system, and more particularly, to a method and an apparatus for transmitting reference signals in uplink MIMO transmission. According to one embodiment of the present invention, the method for transmitting uplink signals through a terminal in the wireless communication system comprises the steps of: receiving control information including the information on a cyclic shift and/or an orthogonal cover code; allocating the multiplexed reference signals onto an uplink subframe; and transmitting the subframe through a multi-antenna. When the uplink MIMO transmission is multiuser MIMO transmission, the reference signals of the terminal and the reference signals of other terminals can be multiplexed by using the orthogonal cover code.

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2010/004180, filed on Jun. 28, 2010,which claims the benefit of earlier filing date and right of priority toKorean Patent Application No. 10-2010-0061030, filed on Jun. 28, 2010,and also claims the benefit of U.S. Provisional Application Ser. No.61/220,595, filed on Jun. 26, 2009, the contents of which are all herebyincorporated by reference herein in their entirety.

TECHNICAL FIELD

The following description relates to a wireless communication systemand, more particularly, to a method and an apparatus for transmittingreference signals in uplink MIMO transmission.

BACKGROUND ART

MIMO (Multi Input Multi Output) is a communication system using aplurality of transmit antennas and a plurality of receiving antennas.The MIMO system may increase channel capacity linearly with the numberof transmit and receiving antennas, without an additional increase infrequency bandwidth. There are two types of MIMO schemes, transmitdiversity and spatial multiplexing. Transmit diversity increasestransmission reliability by transmitting symbols in a plurality ofchannel paths, while spatial multiplexing increases transmission rate bytransmitting different data streams simultaneously through a pluralityof transmit antennas.

MIMO schemes may also be classified into open-loop MIMO and closed-loopMIMO depending on whether a transmitter has knowledge of channelinformation. Open-loop MIMO does not require that the transmitter isaware of channel information. In contrast, the transmitter has channelinformation in closed-loop MIMO. The performance of a closed-loop MIMOsystem depends on how accurate channel information the transmitter gets.

Channel information is information about radio channels between aplurality of transmit antennas and a plurality of receiving antennas(e.g. attenuation, a phase shift, a time delay, etc.). Many stream pathsexists according to the combinations of the transmit and receivingantennas and channel status fluctuates over time in the time andfrequency domains in view of a multipath time delay, which is calledfading, in the MIMO system. Accordingly, a receiver calculates thechannel information through channel estimation. Channel estimation isthe process of estimating channel information required for recovering adistorted transmission signal. For example, the channel estimation isequivalent to estimation of the amplitude and reference phase of acarrier. In other words, the channel estimation is to estimate thefrequency response of a radio link or a radio channel.

For channel estimation, a reference value may be estimated from severalReference Signals (RSs) received from the transmitter using a channelestimator. An RS is symbols transmitted at a high power level withoutcarrying actual data to help channel estimation at the receiver. Boththe transmitter and the receiver may perform channel estimation usingRSs. Specifically, the RS-based channel estimation is to estimate achannel using symbols known to both the transmitter and the receiver andrecover data based on the channel estimate. An RS is called a pilotsignal.

In the meantime, 3rd Generation Partnership Project Long Term Evolution(3GPP LTE) systems are standardized in such a manner that a singleantenna is used for uplink transmission from a User Equipment (UE) to aBS. A demodulation RS (DMRS) based on Cyclic Shift (CS) is defined inuplink single antenna transmission. However, 3GPP LTE-Advanced (LTE-A)systems are required to support multi-antenna transmission even foruplink transmission.

To support uplink MIMO transmission, an RS design scheme is needed whichimproves channel estimation performance for MIMO transmission whilemaintaining subframe design used for single antenna transmissionsupported by the LTE system and backward compatibility.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ona scheme of efficiently designing a DMRS in uplink MIMO transmission,and a method and an apparatus for improving the efficiency of uplinkMIMO transmission by using multiple cyclic shift resources and anorthogonal cover code.

Technical Solution

The object of the present invention can be achieved by providing amethod for transmitting an uplink signal at a terminal in a wirelesscommunication system, the method including: receiving controlinformation including information on Cyclic Shift (CS); multiplexingreference signals for uplink Multiple Input Multiple Output (MIMO)transmission using at least one of the CS or an Orthogonal Cover Code(OCC); allocating the multiplexed reference signals onto an uplinksubframe; and transmitting the subframe through multiple antennas,wherein the reference signals of the terminal and reference signals ofother terminals are multiplexed using the OCC when the uplink MIMOtransmission is multiuser MIMO transmission.

The values of CS may be allocated such that the spacing between CSvalues allocated to two or more ranks has a maximum value.

The reference signals may be precoded reference signals in the case ofspatial multiplexing multi-antenna transmission.

The information on CS may include information on allocation of two CSresources in the case of multi-antenna transmission in a transmissiondiversity scheme.

Information on the OCC may be implicitly acquired by the terminal fromthe information on CS.

The information on the OCC may be received by the terminal through L1/L2control signaling or higher layer signaling.

The control information may be included in a downlink controlinformation format used for scheduling of physical uplink sharedchannels.

The reference signals may be Demodulation Reference Signals (DMRSs).

The object of the present invention can be achieved by providing aterminal transmitting an uplink signal in a wireless communicationsystem, the terminal including: a plurality of antennas; a receivingmodule for receiving a signal from a Base Station (BS) through theplurality of antennas; a transmission module for transmitting a signalto the BS through the plurality of antennas; and a processor forcontrolling the terminal including the plurality of antennas, thereceiving module, and the transmission module, wherein the processor isconfigured to receive control information including information onCyclic Shift (CS) through the receiving module, to multiplex referencesignals for uplink MIMO transmission using at least one of the CS valuesor an orthogonal cover code (OCC), to allocate the multiplexed referencesignals onto an uplink subframe, and transmit the subframe through thetransmission module and the multiple antennas, wherein the referencesignals of the terminal and reference signals of other terminals aremultiplexed using the OCC when the uplink MIMO transmission is multiuserMIMO transmission.

The values of CS may be allocated such that the spacing between CSvalues allocated to two ore more ranks has a maximum value.

The reference signals may bee precoded reference signals in the case ofspatial multiplexing multi-antenna transmission.

The information on CS may include information on allocation of two CSresources in the case of multi-antenna transmission in a transmissiondiversity scheme.

Information on the OCC may be implicitly acquired by the terminal fromthe information on CS.

The information on the OCC may be received by the terminal through L1/L2control signaling or higher layer signaling.

The control information may be included in a downlink controlinformation format used for scheduling of physical uplink sharedchannels.

The reference signals may be Demodulation Reference Signals (DMRSs).

It will be appreciated by person skilled in the art that the objectsthat could be achieved with the present invention are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present invention could achieve will be more clearlyunderstood from the following detailed description.

Advantageous Effects

According to embodiments of the present invention, it is possible toprovide a DMRS which improves the efficiency of uplink MIMO transmissionby using multiple cyclic shift resources and an orthogonal cover code.

It will be appreciated by persons skilled in the art that the effectsthat could be achieved with the present invention are not limited towhat has been particularly described hereinabove and other advantages ofthe present invention will be more clearly understood from the followingdetailed description.

DESCRIPTION OF DRAWINGS

FIG. 1 shows the configuration of a wireless communication system havingmultiple antennas;

FIG. 2 shows channels from a transmit antenna to a receiving antenna;

FIG. 3 shows the configuration of a 3GPP LTE uplink reference signal;

FIG. 4 is a flowchart illustrating a method for transmitting a referencesignal in a terminal according to an exemplary embodiment of the presentinvention;

FIG. 5 is a graph showing BLER simulation results with respect toreference signals according to an exemplary embodiment of the presentinvention;

FIG. 6 is graphs showing FER and MSE simulation results with respect toreference signals according to an exemplary embodiment of the presentinvention; and

FIG. 7 shows the configuration of a terminal according to an exemplaryembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment.

In the embodiments of the present invention, a description is made,centering on a data transmission and reception relationship among a BaseStation (BS) and a User Equipment (UE). The BS is an end node of anetwork, which communicates directly with a terminal. In some cases, aspecific operation described as performed by the BS may be performed byan upper node of the BS.

Namely, it is apparent that, in a network comprised of a plurality ofnetwork nodes including a BS, various operations performed forcommunication with a TERMINAL may be performed by the BS, or networknodes other than the BS. The term ‘BS’ may be replaced with the term‘fixed station’, ‘Node B’, ‘evolved Node B (eNode B or eNB)’, ‘AccessPoint (AP)’, etc. The term “relay” may be used interchangeably with‘Relay Node (RN)’, ‘Relay Station (RS)’, etc. The term ‘terminal’ may bereplaced with the term ‘terminal’, ‘Mobile Station (MS)’, ‘MobileSubscriber Station (MSS)’, ‘Subscriber Station (SS)’, etc.

Specific terms used for the embodiments of the present invention areprovided to help the understanding of the present invention. Thesespecific terms may be replaced with other terms within the scope andspirit of the present invention.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

The embodiments of the present invention can be supported by standarddocuments disclosed for at least one of wireless access systems,Institute of Electrical and Electronics Engineers (IEEE) 802, 3^(rd)Generation Partnership Project (3GPP), 3GPP Long Term Evolution (3GPPLTE), LTE-Advanced (LTE-A), and 3GPP2. Steps or parts that are notdescribed to clarify the technical features of the present invention canbe supported by those documents. Further, all terms as set forth hereincan be explained by the standard documents.

Techniques described herein can be used in various wireless accesssystems such as Code Division Multiple Access (CDMA), Frequency DivisionMultiple Access (FDMA), Time Division Multiple Access (TDMA), OrthogonalFrequency Division Multiple Access (OFDMA), Single Carrier FrequencyDivision Multiple Access (SC-FDMA), etc. CDMA may be implemented as aradio technology such as Universal Terrestrial Radio Access (UTRA) orCDMA2000. TDMA may be implemented as a radio technology such as GlobalSystem for Mobile communications (GSM)/General Packet Radio Service(GPRS)/Enhanced Data Rates for GSM Evolution (EDGE). OFDMA may beimplemented as a radio technology such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, Evolved-UTRA (E-UTRA) etc. UTRA is a partof Universal Mobile Telecommunication System (UMTS). 3GPP LTE is a partof Evolved-UMTS (E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA fordownlink and SC-FDMA for uplink. LTE-A is an evolution of 3GPP LTE.WiMAX can be described by the IEEE 802.16e standard (WirelessMetropolitan Area Network (WirelessMAN-OFDMA Reference System) and theIEEE 802.16m standard (WirelessMAN-OFDMA Advanced System). For clarity,this application focuses on the 3GPP LTE/LTE-A system. However, thetechnical features of the present invention are not limited thereto.

MIMO System Modeling

FIG. 1 shows the configuration of a wireless communication system usingmultiple antennas. As shown in FIG. 1, when the number of transmitantennas and the number of receiving antennas are increased to N_(T) andN_(R) respectively, a channel transmission capacity increases inproportion to the number of antennas in theory, distinguished from acase in which only transmitter or receiver uses multiple antennas.Accordingly, a transmission rate and frequency efficiency can beimproved. The transmission rate can be increased by the product of amaximum transmission rate R₀ when a single antenna is used and a rate ofincrease R_(i) represented by Equation 1 according to increase in thechannel transmission capacity theoretically.R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, a MIMO communication system using four transmit antennasand four receiving antennas can acquire a transmission rate four timesthe transmission rate of a single antenna system in theory. Since thetheoretical capacity increase of the multi-antenna system was proved inthe mid-90s, various techniques for improving a data transfer rate havebeen actively studied and some of the techniques are reflected instandards of wireless communications such as 3^(rd) generation mobilecommunication and next-generation wireless LAN.

MIMO related researches that have been performed so far involveinformation theory researches related to MIMO communication capacitycalculation in various channel environments and multi-accessenvironments, researches on radio channel measurement and modeling,researches on space-time signal processing techniques for improvingtransmission reliability and transmission rate, etc.

A communication scheme in the MIMO system will be described below usinga mathematical model. It is assumed that there are N_(T) transmitantennas and N_(R) receiving antennas in the MIMO system.

Regarding a transmission signal, up to N_(T) pieces of information canbe transmitted through the N_(T) transmit antennas, as expressed as thefollowing vector.s=[s ₁ ,s ₂ , . . . ,s _(N) _(T) ]^(T)  [Equation 2]

A different transmit power may be applied to each piece of transmissioninformation s₁, s₂, . . . , s_(N) _(T) . Let the transmit power levelsof the transmission information be denoted by P₁, P₂, . . . , P_(N) _(T), respectively. Then the transmit power-controlled transmissioninformation ŝ may be given as [Equation 3].ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P_(N) _(T) s _(N) _(T) ]^(T)  [Equation 3]

ŝ may be expressed as a diagonal matrix P of transmit power.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\S_{N_{T}}\end{bmatrix}} = {Ps}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

Let's consider a case in which actual N_(T) transmitted signals x₁, x₂,. . . , x_(N) _(T) are configured by applying a weight matrix W to thetransmit power-controlled information vector ŝ. The weight matrix Wfunctions to appropriately distribute the transmission information tothe antennas according to transmission channel statuses, etc. Thesetransmitted signals x₁, x₂, . . . , x_(N) _(T) are represented as avector X, which may be determined as

$\begin{matrix}\begin{matrix}{X = \begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix}} \\{= {\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1\; N_{T}} \\w_{21} & w_{22} & \ldots & w_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}}} \\{= {W\hat{s}}} \\{= {WPs}}\end{matrix} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

where w_(ij) denotes a weight for a j^(th) piece of information ŝ_(j)transmitted through an i^(th) transmit antenna. W is also referred to asa precoding matrix.

Given N_(R) receiving antennas, signals received at the receivingantennas, y₁, y₂, . . . , y_(N) _(R) may be represented as the followingvector.y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

When channels are modeled in the MIMO wireless communication system,they may be distinguished according to the indexes of the transmitantennas and receiving antennas. A channel between a j^(th) transmitantenna and an i^(th) receiving antenna is represented as h_(ij). It isto be noted herein that the index of the receiving antenna precedes thatof the transmit antenna in h_(ij).

FIG. 2 illustrates channels from N_(T) transmit antennas to an i^(th)receiving antenna. Referring to FIG. 2, the channels from the N_(T)transmit antennas to the i^(th) receiving antenna may be expressed as[Equation 7].h _(i) ^(T) =[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Equation 7]

Hence, all channels from the N_(T) transmit antennas to the N_(R)receiving antennas may be expressed as the following matrix.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1\; N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \lbrack {{Equation}\mspace{14mu} 8} \rbrack\end{matrix}$

Actual channels experience the above channel matrix H and then are addedwith Additive White Gaussian Noise (AWGN). The AWGN n₁, n₂, . . . ,n_(N) _(R) added to the N_(R) receiving antennas is given as thefollowing vector.n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

From the above modeled equations, the received signal is given as

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1\; N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \lbrack {{Equation}\mspace{14mu} 10} \rbrack\end{matrix}$

The above description has been made focusing on a case in which the MIMOcommunication system is used for a single user. However, the MIMOcommunication system can be applied to a plurality of users to acquiremultiuser diversity. This will be briefly described.

A fading channel is known as a main cause of deterioration ofperformance of a wireless communication system. A channel gain varieswith time, frequency and space, and performance deterioration becomesserious as the channel gain decreases. Diversity, one of methods forovercoming fading, uses the fact that there is a very low probabilitythat all independent channels have low gains. There are a variety ofdiversity schemes, one of which is multiuser diversity.

When there are multiple users in a cell, the users have stochasticallyindependent channel gains, and thus the probability that all the channelgains are low is very low. According to information theory, on theassumption that a BS has sufficient transmit power, a total channelcapacity can be maximized by allocating all channels to a user having ahighest channel gain among the multiple users in the cell. The multiuserdiversity can be classified into three schemes.

Temporal multiuser diversity is a scheme of allocating a channel to auser having a maximum channel gain whenever the channel varies withtime. Frequency multiuser diversity is a scheme of allocatingsubcarriers to a user having a maximum gain in each frequency band in afrequency multicarrier system such as Orthogonal Frequency DivisionMultiplexing (OFDM).

If channels vary very slowly in a system using no multicarrier, a userhaving a maximum channel gain will exclusively occupy the channels for along time. Accordingly, other users cannot perform communications. Inthis case, it is necessary to induce the channels to be varied in orderto use the multiuser diversity.

Spatial multiuser diversity uses the fact that users have differentchannel gains according to spaces. An implementation example of thespatial multiuser diversity is Random Beamforming (RBF). The RBF is alsocalled “opportunistic beamforming”. The RBF induces channel variation bybeamforming with an arbitrary weight at a transmitter using multipleantennas.

A description will be given of a Multiuser MIMO (MU-MIMO) scheme whichapplies the multiuser diversity to MIMO.

The MU-MIMO scheme enables various combinations of the number of usersat the transmitter and receiver and the number of antennas of each user.The MU-MIMO scheme is divided into downlink (forward link) and uplink(referees link) which will be respectively described. The downlink meanssignal transmission from a BA to terminals and uplink means signaltransmission from terminals to an BS.

In the case of downlink MU-MIMO scheme, one user can receive signalsthrough N_(R) antennas, and each of N_(R) users can receive signalsusing one antenna in extreme examples. In addition, combinations of thetwo extreme examples are available. That is, a first user uses onereceiving antenna whereas a second user uses three receiving antennas.It is noted that the total number of receiving antennas is N_(R) in allcases. This is called MIMO Broadcast Channel (BC) or Space DivisionMultiple Access (SDMA).

In the case of uplink MU-MIMO scheme, one user can transmit signalsthrough N_(T) antennas and each of N_(T) users can transmit signalsusing one antenna in extreme cases. Further, combinations of the twoextreme examples are available. That is, a certain user uses onetransmit antenna whereas another user uses three transmit antennas. Itis noted that the total number of transmit antennas is maintained asN_(T) in all cases. This is called MIMO Multiple Access channel (MAC).The uplink and downlink have a symmetrical relationship therebetween,and thus a technique used by one side can be used by the other side.

The number of columns and rows of a channel matrix H which indicates achannel state is determined by the number of transmit and receivingantennas. The number of columns in the channel matrix H equals to thenumber of receiving antennas, N_(R), and the number of rows equals tothe number of transmit antennas, N_(T). That is, the channel matrix Hcorresponds to N_(R)×N^(T).

The rank of a matrix is defined as the minimum of the numbers ofindependent rows or columns. Accordingly, the rank of the matrix is notlarger than the number of rows or columns. The rank of the channelmatrix H, rank(H) is limited as follows.rank(H)≦min(N _(T) ,N _(R))  [Equation 11]

If the matrix is eigen value-decomposed, its rank may be defined as thenumber of non-zero eigen values. Similarly, in case of Singular ValueDecomposition (SVD), the rank may be defined as the number of non-zerosingular values. In a physical sense, therefore, the rank of a channelmatrix is the maximum number of different pieces of information that canbe transmitted on given channels.

Reference Signal (RS)

In a wireless communication system, a packet is transmitted on a radiochannel. In view of the nature of the radio channel, the packet may bedistorted during the transmission. To receive the signal successfully,the receiver should compensate for the distortion in the received signalusing channel information. Generally, to enable the receiver to acquirethe channel information, the transmitter transmits a signal known toboth the transmitter and the receiver and the receiver acquiresknowledge of channel information based on the distortion of the signalreceived on the radio channel. This signal is called a pilot signal or areference signal.

In case of data transmission and reception through multiple antennas,knowledge of channel states between transmit antennas and receivingantennas is required for successful signal reception. Accordingly, areference signal should exist separately for each transmit antenna.

Multiplexing is to allocate RSs configured for different antennas to thesame resource area. There are largely three multiplexing schemes, TimeDivision Multiplexing (TDM), Frequency Division Multiplexing (FDM), andCode Division Multiplexing (CDM). Among them, CDM is the process ofmultiplying different code resources allocated to different antennas byRSs for the different antennas in the frequency domain and allocatingthe products to the same radio resources (time/frequency resources).

Configuration of Uplink Reference Signal

FIG. 3 shows the configuration of an uplink RS of 3GPP LTE.

Referring to FIG. 3, a radio frame includes ten subframes. A subframeincludes two slots. A time for transmitting one subframe id defined as aTransmission Time Interval (TTI). In the 3GPP LTE, one subframe may havea length of 1 ms and one slot may have a length of 0.5 ms. However, thestructure of the radio frame and TTI may be varied according to acommunication system.

One slot includes a plurality of SC-FDMA symbols in the time domain andincludes a plurality of resource blocks in the frequency domain. For oneresource block, the horizontal axis thereof represents time axis andvertical axis thereof indicates frequency axis. In the case of normalCP, each slot includes seven symbols. In the case of extended CP, eachslot includes six symbols. The extended CP is generally used in anenvironment having a long delay. In a Single Carrier-Frequency DivisionMultiple Access (SC-FDMA) system, an RS uses all resources of one symbolin order to satisfy the single carrier property. In the 3GPP LTE system,an RS is not precoded on the uplink, distinguished from data, andincludes a Demodulation RS (DMRS) and a Sounding RS (SRS). The DMRS is areference signal for acquiring channel information for demodulation ofuplink data and the SRS is a reference signal used for measurement ofuplink channels. FIG. 3 shows locations of a DMRS and an SRS in the caseof normal CP. The DMRS is allocated to I=3 of slots 1 and 2 andindicated as ‘1’. The SRS is allocated to I=6. Data is allocated to theremaining resource elements. As shown in FIG. 3, One OFDM (or SC-FDMA)symbol in one slot is used to transmit a DMRS. That is, one subframeincludes two slots, data is transmitted on the basis of one subframe asa basic unit, and one subframe is allocated with two DMRSs. Two slotsexisting in one subframe may be located at the same frequency or locatedat different frequencies. Accordingly, the DMRS may have the samefrequency or different frequencies in the first slot and the secondslot. The DMRSs existing in the two slots use the same sequence.

A DMRS sequence and an SRS sequence may be generated using a ConstantAmplitude Zero Autocorrelation Waveform (CAZAC) sequence. The CAZACsequence may be a Zadoff-Chu (ZC) sequence, for example. Various ZCsequences can be generated according to a root index and a cyclic shiftindex. That is, a root index or cyclic shift index can be a seed valueof the ZC sequence. DCI format 0 which is control information for uplinkdata transmission includes a cyclic shift index. A BS can estimatechannels from a plurality of terminals through an orthogonal (orquasi-orthogonal) sequence by allocating different cyclic shift indexesto the terminals.

The length of a DMRS sequence equals to the number of subcarriers of anallocated resource block. A DMRS for a Physical Uplink Shared Channel(PUSCH) is cyclic-shifted. In the LTE system, one slot has one CyclicShift (CS) value. The cyclic shift α in a slot n_(s) is determined basedon Equation 12. RSs from different terminals in one cell can bemultiplexed using CS.α=2πn _(cs)/12  [Equation 12]n _(cs)=(n _(DMRS) ⁽¹⁾ +n _(DMRS) ⁽²⁾ +n _(PRS)(n _(s)))mod 12

where n_(DMRS) ⁽¹⁾ is cell-specifically given according to a parametercyclicShift provided by higher layers. Table 3 illustrates exemplarymapping of the parameter cyclicShift to n_(DMRS) ⁽¹⁾ values. In Equation12, n_(DMRS) ⁽²⁾ is defined by a cyclic shift for a DMRS field ofDownlink Control Information (DCI) format 0 received most recently for atransmission block associated with PUSCH transmission andterminal-specifically given. Table 2 illustrates exemplary mapping ofthe cyclic shift field in DCI format 0 to n_(DMRS) ⁽²⁾ values. InEquation 12, n_(PRS) is a value given by a pseudo-random sequence andhas a pattern hopping to a slot level. The pseudo-random sequence iscell-specifically applied.

TABLE 1 cyclicShift n_(DMRS) ⁽¹⁾ 0 0 1 2 2 3 3 4 4 6 5 8 6 9 7 10

TABLE 2 DCI format 0 n_(DMRS) ⁽²⁾ 000 0 001 6 010 3 011 4 100 2 101 8110 10 111 9

Uplink DMRS Transmission Using Multiantenna

While the following exemplary embodiment of the present invention isdescribed using the LTE-A system, it is noted that it can be applied toany MIMO system according to the same principle.

In the 3GPP LTE system, a terminal supports only one antenna.Accordingly, one antenna is used for the uplink of the LTE system.However, since even the uplink supports MIMO in the LTE-A system, a DMRSused for the uplink needs to be extended. For extension of a DMRSdepending on an MIMO environment, a non-precoded DMRS and a precodedDMRS may be considered. The non-precoded DMRS requires as many DMRSpatterns as the number of antennas as does the conventional downlink.That is, the DMRS pattern should be defined on the basis of the numberof antennas, which can be supported by the system. In the case of theprecoded DMRS, channel information measured at an antenna is multipliedby a precoding matrix and a DMRS pattern is applied to a rankcorresponding to a virtual antenna domain, and thus DMRS overhead can bereduced even when the number of antennas increases. That is, the DMRSpattern of the precoded DMRS should be defined on the basis of thenumber of ranks supported by the system. For example, when the number ofuplink transmit antennas, which can be supported by the system, is 1, 2and 4, the non-precoded DMRS needs to define three patterns and theprecoded DMRS needs to define patterns for ranks 1, 2, 3 and 4. Thefollowing description is given focusing on a case in which the number ofuplink transmit antennas is four. However, the following exemplaryembodiment of the present invention can be applied to any system using aplurality of transmit antennas.

As described above, a DMRS based on a single CS is defined for uplinktransmission in the LTE system based on single antenna transmission. Itis possible to consider a method of applying multiple CS resources to aDMRS as a method for supporting uplink MIMO transmission in the LTE-Asystem while maintaining backward compatibility. A DMRS for each layer(or antenna port) can be code-division-multiplexed using multiple CSresources and transmitted through the same radio resource. At this time,efficient allocation of CS resources is important in order to supportuplink MIMO transmission using multiple CS resource. A detaileddescription will be given of the number of CS resources, which isrequired for each uplink MIMO transmission mode, CS resource separation,and application of an Orthogonal Cover Code (OCC).

Required Number of Cyclic Shift Resources

A variety of transmission schemes for uplink MIMO transmission using upto four transmit (Tx) antennas have been proposed. The number of CSresources for a DMRS depends on a transmission scheme. Table 3illustrates the number of CS resources, required for varioustransmission diversity schemes, and Table 4 illustrates the number of CSresources required for various spatial multiplexing schemes.

TABLE 3 2Tx 4Tx 1 cyclic Slot based PVS, small Slot based PVS, Smalldelay CDD shift delay CDD 2 cyclic STBC, SFBC, FSTD, Large STBC-CDD,SFBC-CDD, shift delay CDD, FSTD-CDD 4 cyclic STBC-FSTD, SFBC-FSTDFSTD,shift Large delay CDD

TABLE 4 2Tx 4Tx 1 cyclic Rank 1 SM with precoded Rank 1 SM with precodedDMRS shift DMRS 2 cyclic Rank 2 SM with Rank 2 with precoded DMRS shiftprecoded DMRS(Using identity matrix) 3 cyclic — Rank 3 SM with precodedDMRS shift 4 cyclic — Rank 3 SM with non-precoded shift DMRSRank 4 SMwith precoded DMRS(Using identity matrix)

Among transmission schemes applicable as a 2 Tx transmission diversityscheme, slot based Precoding Vector Switching (PVS) and small delayCyclic Delay Diversity (CDD) may require one CS resource for a DMRS.Space Time Block Coding (STBC), Space Frequency Block Coding (SFBC),Frequency Switching Transmit Diversity (FSTD) and large delay CDD mayrequire two CS resources for a DMRS.

Among transmission schemes applicable as a 4 Tx transmission diversityscheme, slot based PVS and small delay CCD may require one CS resourcefor a DMRS, STBC-CDD, SFBC-CDD and FSTD-CDD may need two CS resourcesfor a DMRS, and STBC-FSTD, SFBC-FSTD, FSTD and large delay CDD mayrequire four CS resources for a DMRS.

An appropriate number of CS resources can be determined in considerationof LTE design requirements of limitations on the output power of OFDMsymbols in an uplink subframe and allocation of only one symbol per slotfor DMRS transmission. Considering that a lower Peak Power to AverageRatio PAPR is required due to restricted power of a terminal and a highdiversity gain is needed in terms of the necessity of reinforcing linkquality and reducing terminal transmit power, transmission diversityschemes propose supporting up to two CS resources for both 2 Tx antennaand 4 Tx antenna.

In the meantime, in the uplink spatial multiplexing scheme, it issignificant which one of the non-precoded DMRS and precoded DMRS isused. It was proposed that the precoded DMRS be used in the spatialmultiplexing scheme for transmission of ranks 1, 2 and 4, as shown inTable 4. To determine which one of the non-precoded DMRS and precodedDMRS is used for rank-3 transmission, a required number of CS resourcesand complexity of calculation for acquiring precoded channel informationcan be considered.

In the case of a precoded DMRS, the required number of CS resources isdetermined by a transmission rank value instead of the number of uplinktransmit antennas. Accordingly, three CS resources are required forrank-3 transmission. Furthermore, when the precoded DMRS is applied,precoded channel information can be directly acquired.

In the case of a non-precoded DMRS, the required number of CS resourcesequals to the number of uplink transmit antennas. For the non-precodedDMRS, enhanced calculation is needed to acquire precoded channelinformation. Although it is possible to consider a method of using onlytwo CS resources for ranks 3 or more through multiplexing using two CSresources and additional multiplexing using an Orthogonal Cover Code(OCC) in the non-precoded DMRS scheme, this method is disadvantageoussince it requires scheduling restriction (that is, non-slot hoppingPUSCH).

Considering application of the precoded DMRS to other spatialmultiplexing schemes, it is natural for the precoded DMRS to be appliedto a spatial multiplexing scheme for rank-3 transmission. Further, theprecoded DMRS may be advantageous for ease of system implementation.Accordingly, it is suggested that the precoded DMRS is applied to thespatial multiplexing scheme for rank-3 transmission.

Cyclic Shift Resource Allocation

CS separation can be considered to be a main multiplexing scheme in DMRSmultiplexing. In channel estimation based on CDM, it is possible toaccomplish more satisfactory channel estimation performance byallocating spaced CS resources to layers or transmit antennas. Forexample, CS resource separation can be set to 12/N (here, 12 indicatesthe number of CS values for a DMRS and N denotes the number of ranks) inorder to maintain a maximum distance between CS resources according to atransmission rank.

If one terminal performs rank-2 uplink transmission, channels for afirst layer and a second layer can be discriminated from each otherusing DMRS sequences (e.g., ZAZAC sequences) for the first layer and thesecond layer.

In the case of Single User-MIMO (SU-MIMO), if a CS value for one layeris designated, CS values for other layers can be allocated according toa predetermined increment. For example, when CS values are allocated tolayers for rank 2, if a CS value for a first layer is designated as 4,as given by Table 2, 10, spaced from the first layer by 6 (12/2), can beallocated as a CS value for a second layer.

In the case of MU-MIMO, different CS values can be allocated using CSindication bits in a DCI format for uplink transmission. In MU-MIMO,spaced CS resources are required to be allocated between layers of eachterminal since terminals have different timing offsets. Accordingly, itis necessary to allocate CS resources to multiple users each havingmultiple layers more cautiously.

When CS resources are allocated to multiple users having differenttransmission ranks, a fixed increment rule cannot ensure spacing CSresources among the multiple users. Accordingly, CS allocation accordingto a variable increment can be considered for flexibility of CS resourceallocation.

Orthogonal Cover Code

In DMRS multiplexing, orthogonal cover code (OCC) separation among slotsis considered to be a complementary multiplexing scheme. That is, theOCC can be used to increase the capacity of uplink RS resources.

A DMRS (cyclic shifted DMRS) is mapped to one symbol in each of twosymbols which form one subframe (refer to FIG. 3). DMRSs mapped to twosymbols are spread by using a length-2 orthogonal sequence for the twosymbols, which is called orthogonal covering.

The OCC can be used to identify DMRSs for multiple users. For example,when the BS allocates a sequence with CS #0 to a terminal through DCIformat 0, the sequence with CS #0 is used for DMRSs of the first andsecond slots. Here, a positive sign (+) or a negative sign (−) can beallocated to the DMRS sequence of the second slot. If a first user isallocated with CS #0 and a positive-signed OCC (+) and a second user isallocated with CS #0 and a negative-signed OCC (−), DMRSs of the twousers can be distinguished from each other by means of the OCCs. At thistime, since the DMRSs use the same CS resource, the capacity of DMRSresources can be doubled. In the case of an operation using a non-slothopping PUSCH, the capacity of resources can be doubled by applyinglength-2 orthogonal sequence based OCCs to 8 or 12 CS resources. Thelength-2 orthogonal sequence may be a length-2 Walsh sequence (1, 1) and(1, −), for example. To increase the capacity of uplink RS resources bymeans of OCCs, the type of a used OCC can be indicated by allocating anOCC bit field to a control signal for data demodulation. For example,when 1 bit is allocated to an OCC indicator 1, OCC indicators ‘0’ and‘1’ may have the following configuration.

TABLE 5 OCC indicator First slot Second slot 0 1 1 1 1 −1

Further, the bit field included in the control signal when OCCs areapplied to DMRSs may be defined as shown in Table 6.

TABLE 6 Orthogonal Cover Code 1 bit Cyclic Shift value for DMRS 3 bitsAdditional bit field for multi- 0~3 bits antenna/layer

OCCs may be used to increase the spacing between CSs allocated tomultiple antennas (or layers or streams) of a single user.

Different channels are identified by different uplink RSs. In amulti-antenna system, different CS resources may be allocated todifferent antennas (or layers or streams) to distinguish the antennas(or layers or streams) from one another. Channel estimation performanceis improved as the spacing between CS resources increases. As the numberof antennas (or layers or streams) increases, the number of CS resourcesto be allocated to the antennas (or layers or streams) also increases.The resulting reduction of the spacing between CSs may decrease thechannel estimation performance. To avert this problem, an OCC may beapplied to each antenna (or layer or stream). If DMRSs of two slots havethe same frequency, the spacing between CSs increases by means of OCCs.

For example, on the assumption that CS indexes are given as shown inTable 2, if for four antennas, CSs 0, 6, 3 and 9 are respectivelyallocated, the spacing between the CSs for the antennas is 3. Here,DMRSs corresponding to third and fourth antennas are allocated with anegative-signed OCC (−). If length-N sequences with CS #0 are indicatedby (S₀₁, . . . , S_(0N)), (S₀₁, . . . , S_(0N)), (S₆₁, . . . , S_(6N)),(S₃₁, . . . , S_(3N)), (S₉₁, . . . , S_(9N)) are allocated to the DMRSof the first slot. If the DMRS of the second slot uses a negative-signedOCC (−), (S₀₁, . . . , S_(0N)), (S₆₁, . . . , S_(6N)), (−S₃₁, . . . ,−S_(3N)), (−S₉₁, . . . , −S_(9N)) are allocated to the DMRS of thesecond slot. When the DMRSs of the two slots are summed, only thesequences (S₀₁, . . . , S_(0N)), (S₆₁, . . . , S_(6N)) remain with a CSspacing of 6. Likewise, subtraction of the DMRS of the two slots resultsonly in the sequences (S₃₁, . . . , S_(3N)), (S₉₁, . . . , S_(9N)) witha CS spacing of 6. Therefore, the channel estimation performance can beincreased since the spacing between CSs is increased to 6.

For SU-MIMO, OCCs can increase the spacing between CSs according tointer-layer cancellation. However, this advantage can be used only whenthere is a channel estimator which performs a successive inter-layercancellation (SIC) operation.

OCCs may be used to increase the spacing between CSs allocated tomultiple users.

CSs and OCCs may be allocated in consideration of MU-MIMO using multipleantennas. For example, highly dispersive CSs may be allocated to aplurality of antennas (or layers) from the viewpoint of a single user.CS allocation can be performed according to the above-described variousmethods. From the viewpoint of multiple users, however, the spacingbetween CSs of users may be narrowed. This problem can be overcome bymeans of OCCs. When OCCs are applied, the same CS value may be allocatedto a plurality of users according to an OCC type.

Table 7 illustrates an example of applying OCCs to MU-MIMO.

TABLE 7 1^(st) Slot 2^(nd) Slot UE 1 C1 C3 C1 C3 UE 2 C2 C4 −C2 −C4

It is assumed that two terminals respectively transmit two layersthrough MU-MIMO. In this case, the BS is considered to receive fourlayers including the two layers from a first terminal and the two layersfrom a second terminal, and thus the BS is required to recover datacorresponding to the respective layers using four different DMRSs.

The two layers from the first terminal can be multiplexed usingdifferent CS resources C1 and C3. The C1 and C3 can be respectivelygiven CS indexes 0 and 6, for example, to maximize the spacing betweenthe CS resources. The two layers from the second terminal can bemultiplexed using different CS resources C2 and C4. The C2 and C4 can berespectively given CS indexes 3 and 9, for example, to maximize thespacing between the CS resources.

The first and second terminals can be multiplexed using OCCs. OCCs canbe applied to two consecutive slots of one subframe. OCCs {1, 1} can beapplied to the first terminal and OCCs {1, −1} can be applied to thesecond terminal. Accordingly, C2 and C4 corresponding to DMRStransmission symbols of the second slot of the second terminal aremultiplied by −1.

If length-N sequences with CS index k are indicated by (S_Ck_(—)1, . . ., S_Ck_N), (S_C1_(—)1, . . . , S_C1_N), (S_C2_(—)1, . . . , S_C2_N),(S_C3_(—)1, . . . , S_C3_N), (S_C4_(—)1, . . . , S_C4_N) can be appliedto DMRSs of the first slot and (S_C1_(—)1, . . . , S_C1_N), (−S_C2_(—)1,. . . , −S_C2_N), (S_C3_(—)1, . . . , S_C3_N), (−S_C4_(—)1, . . . ,−S_C4_N) can be applied to DMRSs of the second slot.

A description will be given of an operation of the BS for channelestimation by identifying DMRSs for different layer signals from therespective terminals. The BS can receive (S_C1_(—)1, . . . , S_C1_N),(S_C3_(—)1, . . . , S_C3_N) which are obtained by adding up sequencescorresponding to DMRS transmission symbols of the first and secondsymbols to erase sequences using the C2 and C4. The result (S_C1_(—)1, .. . , S_C1_N), (S_C3_(—)1, . . . , S_C3_N) obtained by the BScorresponds to the DMRSs of the first terminal. The two layers of thefirst terminal can be distinguished from each other by means of thedifferent CS resources C1 and C3. In addition, the BS can receive(S_C2_(—)1, . . . , S_C2_N), (S_C4_(—)1, . . . , S_C4_N) which areobtained by performing subtraction of the sequences corresponding to theDMRS transmission symbols of the first and second symbols to erasesequences using the C1 and C3. The result (S_C2_(—)1, . . . , S_C2_N),(S_C4_(—)1, . . . , S_C4_N) obtained by the BS corresponds to the DMRSsof the second terminal. The two layers of the second terminal can bedistinguished from each other by means of the different CS resources C2and C4.

This MU-MIMO scheme using OCCs can remarkably reduce interferences amongmultiple users.

Whether or not an OCC is used can be indicated to a specific terminal bydefining an OCC indication bit through L1/L2 signaling (e.g., apredetermined PDCCH or a PUSCH in the form of a MAC message) or higherlayer signaling (e.g., RRC signaling). Otherwise, whether or not an OCCis used can be indicated to a specific terminal through newinterpretation of other parameters of L1/L2 signaling.

Alternatively, whether or not an OCC is used can be indicated to aspecific terminal without using additional L1/L2 signaling and/or RRCsignaling. For instance, an OCC can be indicated to the TERMINAL using aDMRS field of DCI format 0 which informs the terminal of a CS index.That is, the specific terminal can be informed of CS resources allocatedthereto with information on whether OCCs are used, and can recognizethat OCCs are used when reserved CS resources (e.g., 1, 5 and 7) orunused CS indexes (e.g., −2 and −4) are allocated thereto.

A description will be given of a method for transmitting a DMRS in aterminal according to an exemplary embodiment of the present inventionwith reference to FIG. 4.

The terminal may receive control information including informationregarding CS resource allocation from the BS (S410). This controlinformation may be CS information about a DMRS field of DCI format 0.The terminal may receive information on OCCs and implicitly acquire theinformation on OCCs from the CS information.

The terminal may multiplex DMRSs using CS resources allocated theretoand predetermined OCCs (S420). Specifically, the terminal cancode-division-multiplex DMRSs of a plurality of layers (or antennaports) using different CS resources. In addition, the terminal cancode-division-multiplex the DMRSs using OCCs.

In the case of MU-MIMO, OCCs can be used as code resources formultiplexing DMRSs of terminals. For instance, if first and secondterminals transmit signals to the BS through MU-MIMO, OCCs {1, 1} areapplied to cyclic shifted DMRSs of the first terminal and OCCs {1, −1}are applied to cyclic shifted DMRSs of the second terminal such that theDMRSs of the first and second terminals can be multiplexed by means ofthe OCCs.

The terminal may allocate the multiplexed DMRSs onto an uplink subframe(S430) and transmit the subframe through multiple antennas (S440). TheDMRSs may be mapped to the fourth symbol of each slot in the case ofnormal CP, as shown in FIG. 3.

Performance for Precoded DMRS

A description will be given of results of measurements of Block ErrorRate (BLER) for precoded DMRSs in the case of a spatial multiplexingscheme with 4 Tx antennas and rank 3 with reference to FIG. 5.

On the assumption that output power of OFDM symbols of uplink subframesis restricted and only one symbol per slot is allocated for DMRStransmission, performance measurement was performed focusing on OCCs andpotential channel estimation performance of an SIC channel estimator.For uplink precoded DMRSs, OCCs were applied to a second layer. Sixrepetitions were assumed for the SIC channel estimator. Simulationassumption and parameters are illustrated in Table 8.

TABLE 8 Parameter Assumption Multiple Access Scheme pure SC-FDMA CarrierFrequency 2 GHz System Bandwidth 5 MHz Subframe length 1.0 ms ResourceAllocation Localized Mode 5 RBs Frequency Hopping Non-Slot HoppingModulation and Coding Rate QPSK ½, 16QAM ½, 16QAM ¾ Channel Coding Turbocode: max-log-MAP Channel Models SCM-C (X pol) Mobile Speed (km/h) 3km/h Channel Estimation DFT based channel estimation Antennaconfiguration 4 transmitter and 4 receiver (4Tx, 4Rx) Number of Transmitrank Rank 3 Number of codeword 2 Layer mixing SC-FDM symbol levelTransmission Scheme Closed-loop Spatial Multiplexing Codebook Cubicmetric friendly codebook for rank 3 [7] Precoding Single PMI PMI updateperiod 2 TTI Receiver Type MMSE receiver Cyclic shift value index 0, 2,4

Left curves of the graph of FIG. 5 show results in the case in whichDMRS multiplexing using 3 CS resources was applied to Quadrature PhaseShift Keying (QPSK) modulation scheme and DMRSs were precoded (W) andthe case in which the DMRS multiplexing was performed and DMRSs werenon-precoded. Right curves of the graph of FIG. 5 show results in thecase in which DMRS multiplexing using 3 CS resources was applied to 64Quadrature Amplitude Modulation (QAM) and DMRSs were precoded (W) andthe case in which the DMRS multiplexing was performed and DMRSs werenon-precoded.

As shown in FIG. 5, in the BLER performance of rank-3 spatialmultiplexing using precoded DMRSs and channel estimation, the OCCs andSIC channel estimator do not largely affect the performance in alow-order modulation scheme whereas DMRS multiplexing using OCCsimproves channel estimation performance in a high-order modulationscheme. Particularly, an OCC gain is decreased to below 10% error ratewhen the SIC channel estimator is used.

Cyclic Shift Separation Between Layers/Antennas

A description will be given of results of link level simulations inuplink SU-MIMO according to spacing between CSs with reference to FIGS.6A and 6B. Simulation parameters are shown in Table 9.

TABLE 9 Parameters Value Carrier Frequency 2 GHz # of used RB 3RB (36subcarriers) Error Correction Coding 3GPP Turbo Code Rate ½ ModulationQPSK TERMINAL Velocity 3 km/h Channel Model TU 6-ray Channel EstimationDFT based channel estimation Number of Receive Antennas 2 Number ofTransmit Antennas 2 Precoder Identity 2 × 2

In FIGS. 6A and 6B, two cases Case A and Case B are assumed for thespacing between CSs. Case A is a case in which the spacing between CSsfor DMRSs of layers corresponds to half an OFDM symbol length, whichrepresents that orthogonal frequency codes are arranged over twoconsecutive subcarriers in the frequency domain. That is, this meansthat granularity of channel estimation is one per two subcarriers,approximately. Case B is a case in which the spacing between CSs forDMRSs of layers corresponds to ⅙ of the OFDM symbol length.

In the simulations, Frame Error Rates (FERs) and Means Square Error(MSEs) of Case A and Case B were compared with each other. FIG. 6A showsFERs for an average between slots and one slot in both Case A and Case Band FIG. 6B shows MSEs for an average between slots and one slot in bothCase A and Case B.

Considering the spacing between CSs, the FER of Case A is similar tothat of Case B in a low-order Modulation and Coding Scheme (MCS) such asQPSK 1/2. Comparing Case A to Case B, MSE is degraded as a CS separationdegree increase. The influence of MSE degradation on the performancebecomes severe as orders of SNR, MCS and MIMO increase. Therefore, it isdesirable that CSs should be allocated to layers such that the spacingbetween CSs becomes a maximum value in uplink SU-MIMO.

As described above through the embodiments, the following can beconsidered in DMRS design for uplink transmission diversity and spatialmultiplexing schemes in order to maximize the efficiency of uplink MIMOtransmission. Up to two DMRS CS resources can be used for both 2 Txantennas and 4 Tx antennas of uplink transmission diversity schemes.Highly dispersive CS resources can be preferably selected. PrecodedDMRSs can be used for 4-Tx-antenna rank-3 SU-MIMO transmission in uplinkspatial multiplexing schemes. DMRS multiplexing can be supported thespacing between CSs can be increased using OCCs among slots. Inaddition, interferences in multiplexing of a plurality of MU-MIMOterminals can be reduced using OCCs.

FIG. 7 shows the configuration of a terminal according to an exemplaryembodiment of the present invention.

Referring to FIG. 7, the terminal includes a transmission module 710, areceiving module 720, a processor 730, a memory 740, and an antennamodule 750.

The transmission module 710 may transmit signals, data and informationto the BS over uplink. The receiving module 720 may receive signals,data and information from the BS over downlink. The processor 730 maycontrol the overall operation of the TERMINAL including transmission andreception of signals, data and information through the transmissionmodule 710 and the receiving module 720. The antenna module 750 mayinclude a plurality of antennas. MIMO can be supported if at least oneof a transmitter and a receiver includes a plurality of antennas.

The processor 730 may be configured to receive control informationincluding information about CS resources through the receiving module720, multiplex DMRSs for uplink MIMO transmission using the CS valuesand/or OCCs, allocate the multiplexed DMRSs onto an uplink subframe, andtransmit the subframe through the transmission module 710 and theantennas of the antenna module 750. Here, if the uplink MIMOtransmission corresponds to MU-MIMO transmission, DMRSs of the terminaland DMRSs of other terminals can be multiplexed by using OCCs.

In addition, the processor 730 may process information received by theterminal and information to be transmitted from the terminal. The memory740 may store the processed information for a predetermined time and maybe replaced by a component such as a buffer (not shown).

The embodiments of the present invention can be implemented by variousmeans. For example, the embodiments of the present invention can beimplemented by hardware, firmware, software, or combination thereof.

In a hardware configuration, the embodiments of the present inventionmay be implemented by one or more ASICs (Application Specific IntegratedCircuits), DSPs (Digital Signal Processors), DSPDs (Digital SignalProcessing Devices), PLDs (Programmable Logic Devices), FPGAs (FieldProgrammable Gate Arrays), processors, controllers, microcontrollers,microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention can be implemented by a type of a module, a procedure, or afunction, which performs functions or operations described above.Software code may be stored in a memory unit and then may be executed bya processor. The memory unit may be located inside or outside theprocessor to transmit and receive data to and from the processor throughvarious means which are well known.

The detailed description of the preferred embodiments of the presentinvention has been given to enable those skilled in the art to implementand practice the invention. Although the invention has been describedwith reference to the preferred embodiments, those skilled in the artwill appreciate that various modifications and variations can be made inthe present invention without departing from the spirit or scope of theinvention described in the appended claims. For example, an embodimentof the present invention may be constructed by combining components orconfigurations of the above-described embodiments of the presentinvention. Accordingly, the invention should not be limited to thespecific embodiments described herein, but should be accorded thebroadest scope consistent with the principles and novel featuresdisclosed herein.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. In addition, it will be obvious to those skilled inthe art that claims that do not explicitly cite in each other in theappended claims may be presented in combination as an exemplaryembodiment of the present invention or included as a new claim bysubsequent amendment after the application is filed.

INDUSTRIAL APPLICABILITY

The above embodiments have been described, focusing on 3GPP LTE systems.However, the present invention is not limited thereto and can be appliedin the same manner to methods for transmitting reference signals in avariety of mobile communication systems to which MIMO is applied.

The invention claimed is:
 1. A method for transmitting an uplink signalat a terminal in a wireless communication system, the method comprising:receiving, by the terminal from a base station, control information fora plurality of layers or antennas consisting of a first layer orantenna, a second layer or antenna, a third layer or antenna, and afourth layer or antenna; generating, by the terminal, a plurality ofreference signals for uplink Multiple Input Multiple Output (MIMO)transmission using the control information; precoding, by the terminal,the plurality of reference signals for spatial multiplexing; andtransmitting, by the terminal to the base station, the plurality ofprecoded reference signals via an uplink subframe via the plurality oflayers or antennas, the uplink subframe consisting of two slots, whereinthe control information indicates that: a first layer, a second layer, athird layer, and a fourth layer of the plurality of reference signalsuse an OCC value of 1 in the first slot of the uplink subframe; thefirst and second layers of the plurality of reference signals use theOCC value of 1 in the second slot of the uplink subframe; and the thirdand the fourth layers of the plurality of reference signals use an OCCvalue of −1 in the second slot of the uplink subframe, and wherein thecontrol information further indicates that: the first layer of theplurality of reference signals uses a CS value of 0; the second layer ofthe plurality of reference signals uses a CS value of 6; the third layerof the plurality of reference signals uses a CS value of 3; and thefourth layer of the plurality of reference signals uses a CS value of 9.2. The method according to claim 1, wherein: the CS and OCC values forthe first and second layers of the plurality of reference signals areused by the terminal; and the CS and OCC values for the third and fourthlayers of the plurality of reference signals are used by anotherterminal.
 3. The method according to claim 1, wherein the information onthe OCC is implicitly acquired by the terminal from the information onthe CS.
 4. The method according to claim 1, wherein the information onthe OCC is received by the terminal through L1/L2 control signaling orhigher layer signaling.
 5. The method according to claim 1, wherein thecontrol information is included in a downlink control information formatused for scheduling of physical uplink shared channels.
 6. The methodaccording to claim 1, wherein the plurality of reference signals areDemodulation Reference Signals (DMRSs).
 7. A terminal transmitting anuplink signal in a wireless communication system, the terminalcomprising: a plurality of layers or antennas, the plurality of layersor antennas consisting of a first layer or antenna, a second layer orantenna, a third layer or antenna, and a fourth layer or antenna; areceiving configured to receive a signal from a Base Station (BS) viathe plurality of layers or antennas; a transmitter configured totransmit a signal to the BS via the plurality of layers or antennas; anda processor configured to control the terminal including the pluralityof layers or antennas, the receiver and the transmitter, wherein theprocessor is further configured to: control the receiver to receive,from the BS, control information including information for the pluralityof layers or antennas; generate a plurality of reference signals foruplink Multiple Input Multiple Output (MIMO) transmission using thecontrol information; precode the plurality of reference signals forspatial multiplexing; and control the transmitter to transmit, to theBS, the plurality of precoded reference signals via an uplink subframevia the plurality of layers or antennas, the uplink subframe consistingof two slots, wherein the plurality of reference signals are precodedreference signals for spatial multiplexing of the plurality of layers orantennas, wherein the control information indicates that: a first layer,a second layer, a third layer, and a fourth layer of the plurality ofreference signals use an OCC value of 1 in the first slot of the uplinksubframe; the first and second layers of the plurality of referencesignals use the OCC value of 1 in the second slot of the uplinksubframe; and the third and fourth layers of the plurality of referencesignals use an OCC value of −1 in the second slot of the uplinksubframe, and wherein the control information further indicates that:the first layer of the plurality of reference signals uses a CS value of0; the second layer of the plurality of reference signals uses a CSvalue of 6; the third layer of the plurality of reference signals uses aCS value of 3; and the fourth layer of the plurality of referencesignals uses a CS value of
 9. 8. The terminal according to claim 7,wherein: the CS and OCC values for the first and second layers of theplurality of reference signals are used by the terminal; and the CS andOCC values for the third and fourth layers of the plurality of referencesignals are used by another terminal.
 9. The terminal according to claim7, wherein the information on the OCC is implicitly acquired by theterminal from the information on the CS.
 10. The terminal according toclaim 7, wherein the information on the OCC is received by the terminalthrough L1/L2 control signaling or higher layer signaling.
 11. Theterminal according to claim 7, wherein the control information isincluded in a downlink control information format used for scheduling ofphysical uplink shared channels.
 12. The terminal according to claim 7,wherein the plurality of reference signals are Demodulation ReferenceSignals (DMRSs).
 13. The method according to claim 1, wherein theplurality of reference signals are mapped to only one Single CarrierFrequency Division Multiple Access (SC-FDMA) symbol for each slot of theuplink subframe.
 14. The method according to claim 13, wherein theplurality of reference signals for the plurality of layers or antennasare mapped to same consecutive subcarriers.
 15. The terminal accordingto claim 7, wherein the plurality of reference signals are mapped toonly one Single Carrier Frequency Division Multiple Access (SC-FDMA)symbol for each slot of the uplink subframe.
 16. The terminal accordingto claim 15, wherein the plurality of reference signals for theplurality of layers or antennas are mapped to same consecutive subcarriers.